Cardioid beamformer with noise reduction

ABSTRACT

A method is provided for detecting and localizing acoustic sources.  Acous signals are transmitted by a sonobuoy and return signals are received. Electrical signals, corresponding to the received return signals, are multiplexed and transmitted. The transmitted signals are received by a receiver which is separated from the transmitter. The receiver applies the received signal to a demultiplexer which is separate from the the receiver. The demultiplexer demultiplexes the applied signals to provide electrical signals to the cardioid beamformer representative of the sonar return signals.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of acoustic underwater listeningdevices and in particular to an acoustic underwater water listeningdevice having directional and omnidirectional hydrophones for detectingand localizing acoustic sources.

2. Background Art Statement

A sonobuoy is a passive, directional device used for the purpose ofdetecting and localizing a target in water. A passive sonobuoy detectsunderwater sounds, converts them to electrical energy, and transmits toa receiving station a signal representative of the underwater sounds.

SUMMARY OF THE INVENTION

A method is provided for detecting and localizing acoustic sources.Acoustic signals transmitted in water are received by a sonobuoy.Electrical signals, corresponding to the received return signals, aremultiplexed and transmitted. The transmitted signals are received by areceiver which is separated from the transmitter. The receiver appliesthe received signal to a demultiplexer which is separate from the thereceiver. The demultiplexer demultiplexes the applied signals to provideelectrical signals representative of the sonar return signals to thecardioid beamformer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram representation of the cardioid acousticbeamformer system of the present invention.

FIGS. 2a-d show graphical representations of polar data measured byhydrophones of a hydrophone array associated with the cardioid acousticbeamformer system of FIG. 1.

FIG. 3 shows a schematic representation of the electronic circuitry ofthe cardioid system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown a block representation ofcardioid acoustic beamformer system 36 of the present invention.Cardioid acoustic beamformer system 36 uses data from a conventionalthree-hydrophone array (not shown) which is arranged in a conventionalmanner which is well understood by those skilled in the art. Onehydrophone of the three-hydrophone array is omnidirectional as shown inblock 24 of system 10. The remaining two hydrophones of thethree-hydrophone array of acoustic beamformer system 36 are directionalas shown in block 20. Cardioid acoustic beamformer system 36 produces anoutput that permits computation of the bearing of an acoustic sourcerelative to the position of the three-hydrophone array, in addition toconventional detection of the object which is the acoustic source. Thebearing output of the object is provided by acoustic beamformer system36 in combination with the three-hydrophones of the array and a magneticcompass as shown in block 26 to determine azimuth angle of the objectwith respect to magnetic north.

Each hydrophone of the three-hydrophone array produces its own outputwhich is transmitted by radio frequency to receiving station 38. Thisdata is transmitted by way of cable 16 and radio transmitter 14, havingvery high frequency antenna 12. The information from thethree-hydrophone array is multiplexed onto single cable 16, as shown inblock 18, and relayed in a multiplexed manner to receiving station 38.The received data is recorded and demultiplexed by demultiplexer 34 foranalysis at receiving station 38.

Demultiplexer 34 and cardioid beamformer 36 are provided as two separateunits in the system of the present invention. Thus the demultiplexedsignals from demultiplexer 34 are applied to cardioid 36 bydemultiplexer means 34 by way of three demultiplexed signal lines 35.The demultiplexed data of the sonobuoy applied to cardioid beamformer 36is an electronic representation of acoustic signals received by thethree-hydrophone array from all directions. The three-dimensionalinformation is spherical in shape with the omnidirectional hydrophone atthe center. The data from the two directional hydrophones enable thedirection of arrival of acoustic signals to be determined using cardioidacoustic beamformer system 36.

Additionally, cardioid beamformer system 36 uses this data from thedirectional hydrophones of system 10 to reduce noise arriving from otherdirections relative to the signal direction. Demultiplexed data from thetwo directional hydrophones used in sonobuoy system 36 are termed COSand SIN. The COS hydrophone data is an electronic description of signalsreceived from the north and south directions relative to sonobuoy system10. It is representative of a dumbbell shaped volumetric figure eightshape whose axis is the north-south axis and is centered around the COShydrophone. The SIN data received from the other directional hydrophonerepresents acoustic signals received from east and west directions, alsoin a volumetric figure eight shape with its axis aligned with theeast-west axis.

A geometric cardioid is, in general, the graph of a polar equation ofthe forms shown in Equations (1) plotted in polar coordinates, where xis a real number. Plotted for θ from zero to three hundred sixtydegrees, the following polar Equations (1) produce heart-shaped orcardioid graphs.

    r.sub.1 =x(1-cos θ)                                  Equations (1)

    r.sub.2 =x(1+cos θ)

    r.sub.3 =x(1-sin θ)

    r.sub.4 =x(1+sin θ).

Setting x equal to unity and substituting OMNI for the number 1, COS forcos θ, and SIN for sin θ, polar Equations (2), of a form similar to theform of polar Equations (1), can be determined as follows:

    r.sub.1 =OMNI-COS                                          Equations (2)

    r.sub.2 =OMNI+COS

    r.sub.3 =OMNI-SIN

    r.sub.4 =OMNI+SIN.

Referring now to FIGS. 2a-d, there are shown polar plots of laboratorydata measured at the outputs of the cardioid acoustic beamformer system36 for a single frequency. The polar plots of data are illustrated asgraphical representations 40, 42, 44, and 46. These graphicalrepresentations correspond to three-dimensional heart shaped receivingpatterns with a null in each of the cardinal headings. These shapes aremathematically expressed by Equations (2). The forms of Equations (2)are representative of the forms of Equations (1). Furthermore, usingaddition and subtraction circuits, Equations (2) may be realized usingthe outputs of cardioid acoustic beamformer system 36. Thus cardioidshaped curves may be obtained from the data provided by cardioidacoustic beamformer system 36. However, the data of cardioid acousticbeamformer system 36 is three-dimensional rather than two-dimensional asrepresented by Equations (2). This permits eliminating or discriminatingdata reception from a specified direction.

For example, if an object is at an orientation south of receivingsonobuoy system 10, cardioid beamformer system 36 may null signals whichare determined to be extraneous noise from the north direction byselective orienting of the null of the heart-shaped cardioid beampattern toward the north. It is thus possible to obtain a representationupon which to base judgment compared to only an OMNI representation.Also, by eliminating data, or null steering, from the east or westdirections, it is possible to determine that an object in a moresoutheast or southwest direction with a higher degree of confidence.

Returning to Equations (2), the r₁ cardioid is termed N, the r₂ cardioidis termed S, the r₃ cardioid is termed E, and the r₄ cardioid is termedW. For navigational acoustic work, zero degrees is referenced tomagnetic north and the cardioids obtain their directional name forbearing from the direction of the null or absence of data associatedwith the cardioid shape. Degree determinations, therefore, are asfollows: N is zero degrees, E is ninety degrees, S is one hundred eightydegrees, and W is two hundred seventy degrees. Information from sonobuoysystem 10 is demultiplexed and regenerated by electronically summing anddifferenciating the OMNI, COS and SIN signals to represent theanalytical relationships of Equations (2) in acoustic beamformer system36.

Referring now to FIG. 3, there is shown a schematic representation ofthe electronic circuitry of cardioid 36 which processes data transmittedfrom sonobuoy system 10. To electronically sum and difference the OMNI,COS and SIN signals, conventional operational amplifiers connected toform a conventional summing amplifier configuration. The output of aconventional operational amplifier connected as the first of twooperational amplifiers forming the summing amplifier configuration ismathematically represented as: ##EQU1## where R₂ is a feedback resistervalue located between the output and the inverting input terminal of theoperational amplifier, R₁ is the input resistor value. The resistor isthe same value between each voltage to be summed, V₁ and V₂, and theinverting input terminal of the first operational amplifier. The minussign before the expression of Equation (3) indicates that the output ofthe first operational amplifier is one hundred eighty degrees out ofphase with the input of the first operational amplifier. Such anoperational amplifier is thus an inverting amplifier.

Because the first operational amplifier of the summing amplifierconfiguration is an inventing amplifier, the second operationalamplifier of the summing amplifier configuration is also an invertingamplifier. The second operational amplifier of the summing amplifier hasan output value of: ##EQU2## where V_(IN) is the inverted output voltageof the first summing operational amplifier. The resistance of resisterR₃ is equal to the resistance of resister R₄ in order to provide unitygain in the second inventing amplifier. The second inversion provided bythe second operational amplifier of the summing amplifier pair createsan output with the same phase as the summing amplifier configurationinput. Making the resistance of resister R₁ equal to the resistances ofresisters R₂, R₃, and R₄ permits an ideally achievable gain of unityusing two operational amplifiers for this operation. The values V₁ +V₂of the summing amplifier configuration, from Equation (3), representOMNI+COS, S, and OMNI+SIN, W, values from cardioid acoustic beamformersystem 36.

Similarly, two differences amplifiers may be coupled wherein the outputsare represeted by ##EQU3## where V₂ -V₁ denotes OMNI-COS, N, andOMNI-SIN, E, values from cardioid acoustic beamformer system 10. Onlyone operational amplifier per difference amplifier is needed since thevoltage output phase is not changed from the input. From the differenceamplifier Equation (4), R₂ assumes the feedback resister and the pulldonresister, located from the non-inverting input to ground, are the samevalue. R₁ assumes that the input register between V₁ and the invertinginput and the resister between V₂ and the non-inverting input are thesame value. Thus, making R₂ equal R₁, unit gain may be achieved. Allvalues of resisters used in the summing and differenciating circuits maybe ten kiloohms.

Therefore, Equation (2) are realized using the circuitry of FIG. 3. TheSIN signal is applied to input terminal 120 of cardioid 36. Similarly,the OMNI input is applied to input terminal 122 and the COS input isapplied to input terminal 124. The SIN input, received by way of inputterminal 120, is subtracted from the OMNI signal received by Way ofinput 122 in operational amplifier 100. Thus, output terminal 102 ofcardioid 36 provides the OMNl-SlN of polar Equations (2).

The COS input, received by way of input terminal 124, is subtracted fromthe OMNI signal, received by way of input terminal 122, by differentialapplifier 104. Thus output terminal 106 of cardioid 36 provides theOMNI-COS term of polar Equations (2). The OMNI and SIN inputs are addedin differential applifier 108. Thus output terminal 110 of cardioid 36provides the OMNI-SIN term of polar Equation (2). The OMNI and COSinputs are added in differencial amplifier 112. Thus the OMNI+COS termof polar Equations (2) is provided at output terminal 114 of Cardioid36.

Many modifications and variations of present invention are possible inview of the above disclosure. It is therefore to be understood, thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

We claim:
 1. A method for detecting and localizing acoustic sources,comprising the steps of:(a) providing a plurality of first electricalsignals representative of sonar signals; (b) receiving a plurality ofsecond electrical signals representative of sonar return signalscorresponding to said first electrical signals; (c) multiplexing saidsecond electrical return signals; (d) transmitting said multiplexedelectrical return signals; (e) receiving said transmitted multiplexedsignals by multiplexed signal receiving means separate from saidtransmitting means; (f) applying, by said multiplexed signal receivingmeans, said received signals to demultiplexing means for demultiplexingof said signal to provide a demultiplexed signal; (g) applying ademultiplexed signal by signal line means, coupled to saiddemultiplexing means to cardioid means separated from saiddemultiplexing means; and, (h) processing said applied demultiplexedsignal by said cardioid means to provide electrical signalsrepresentative of said sonar return signals.
 2. The method for detectingand localizing acoustic sources of claim 1, wherein step (b) comprisesreceiving said second electrical signals by means of hydrophone means.3. The method for detecting and localizing acoustic sources of claim 2,wherein step (b) comprises receiving said second electrical signals bymeans of a hydrophone array.
 4. The method for detecting and localizingacoustic sources of claim 3, wherein step (b) comprises receiving saidsecond electrical signals by means of a three-dimensional hydrophonearray.
 5. The method for detecting and localizing acoustic sources ofclaim 3, wherein at least one hydrophone of said array is anomnidirectional hydrophone.
 6. The method for detecting and localizingacoustic sources of claim 5, wherein at least one hydrophone of saidhydrophone array is directional.
 7. The method for detecting andlocalizing acoustic sources of claim 6, comprising the further step ofdetermining the direction of at least one of said sonar return signalsby means of said directional hydrophone.
 8. The method for detecting andlocalizing acoustic sources of claim 1, wherein step (i) comprisescombining said demultiplexed signals.
 9. The method for detecting andlocalizing acoustic sources of claim 8, wherein said combining of saiddemultiplexed signals comprises adding and substracting saiddemultiplexed signals.
 10. A system for detecting and localizingacoustic sources, comprising;means for providing a plurality of firstelectrical signals representative of sonar signals; first means forreceiving a plurality of second electrical signals representative ofsonar return signals corresponding to said first electrical signals;means for multiplexing said second electrical return signals; means fortransmitting said multiplexed electrical return signals; second meansfor receiving said transmitted multiplexed signals, said first receivingmeans being separated from said transmitting means; means, coupled tosaid first receiving means, for applying said received signals todemultiplexing means for demultiplexing of said signal to provide ademultiplexed signal; signal line means coupled to said demultiplexingmeans, for applying said demultiplexed signal to cardioid meansseparated from said demultiplexing means; and means within said cardioidmeans for processing said applied demultiplexed signal by said cardioidmeans to provide electrical signals representative of said sonar returnsignals.
 11. The system for detecting and localizing acoustic sources ofclaim 10, wherein said first means for receiving said first electricalsignals comprises hydrophone means.
 12. The system for detecting andlocalizing acoustic sources of claim 11, wherein said hydrophone meanscomprises a hydrophone array.
 13. The system for detecting andlocalizing acoustic sources of claim 12, wherein said hydrophone arraycomprises a three-dimensional hydrophone array.
 14. The system fordetecting and localizing acoustic sources of claim 13, wherein at leastone hydrophone of said hydrophone array comprises an omnidirectionalhydrophone.
 15. The system for detecting and localizing acoustic sourcesof claim 14, wherein at least one hydrophone of said hydrophone array isdirectional.
 16. The system for detecting and localizing acousticsources of claim 15, further comprising means for determining thedirection of said sonar return signals by means of said directionalhydrophone.
 17. The system for detecting and localizing acoustic sourcesof claim 10, wherein said means for processing said demultiplexed signalcomprises means for combining said demultiplexed signals.
 18. The systemfor detecting and localizing acoustic sources of claim 17, wherein saidmeans for combining said signal comprises means for adding andsubtracting said signal.